With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, and intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the is intermediate pressure compressor 14 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
It is frequently necessary to effect a seal in a clearance gap between two components that are capable of relative movement. In particular seals are often required to seal between a rotatable shaft and a housing surrounding the shaft, the housing having a bore through which the shaft extends.
Within a gas turbine engine, the interconnecting shafts (ie which extend between the fan 12 or a compressor 13, 14 and the respective turbine 18, 17, 16) rotate at relatively high speeds and are exposed to pressurised hot gases. Seals are required for these shafts and the seal performance can have a significant effect on the overall efficiency of the gas turbine engine. There are a number of seal designs that have been proposed for such purposes and that are designed to operate within the harsh environment found within a gas turbine engine.
For example, U.S. Pat. No. 3,917,150 proposes a brush seal, which is a seal for concentric shafts and comprises a pack of flexible wires or bristles supported by a solid backing ring. The bristles contact the rotating shaft, and provide a restriction to the axial fluid flow along the shafts. The bristles are inclined to the radial direction so as to deflect under any shaft movement, forming a compliant seal. The bristles tend to blow down onto the rotor, further reducing leakage.
U.S. Pat. Nos. 6,267,381 and 6,343,792 propose leaf seals, which are also for sealing concentric shafts. The leaf seal comprises a stack of sheet like elements which are inclined to the radial direction, and offer a compliant seal with higher axial rigidity. Unlike the brush seal, the leaf pack may be enclosed by cover plates at the upstream and downstream sides of the seal. The cover plates can be used to control the pressure distribution in the pack, and hence leaf blow-down or lift-up.
In general, there is a demand for robust seal designs with low leakage characteristics to reduce specific fuel consumption in gas turbines. In respect of rotating shafts, this typically means running annular seals in close proximity to (and often in contact with) the shaft, resulting in high wear and frictional heat generation, with adverse consequences for part life and engine efficiency.
High leakages and hence performance penalties are also often associated with linear or slot geometries at various interfaces in the engine, including between surfaces which are not relatively rotating.
Reducing leakages at both rotary and non-rotary sealing locations can contribute to reduced specific fuel consumption.